Heretofore the prior art has suggested the use of elastomeric polyurethane as suitable materials for the manufacture of cast solid and pneumatic tires. The long chain polyurethane polymers useful in accordance with the prior art and useful in accordance with the present invention are linear urethane rubbery polymers prepared by reacting a generally linear polyester or polyether, or a combination of these materials, with one or more organic diisocyanates, usually an aromatic diisocyanate such as toluene diisocyanate. Reference may be had to the U.S. Pat. No. to Gruber et al 3,225,007 which discloses polyurethanes of the type useful in accordance with the present invention and their use in the manufacture of cast articles.
It has been found that with cast articles formed from linear polyurethanes of the polyether, polyester, or mixed polyether-polyester types, when such articles are submitted to high shear conditions, there is a tendency for the polymer to melt and become slippery. The risks involved in utilization of such materials in automotive vehicle tires, for example, are evident.
It has been found that linear elastomeric polyurethanes may be improved in respect of this and other properties by dispersing into the polyurethane matrix from 5 to 35% by weight of a nonfunctional hydrocarbon rubber together with a curing agent for the polyurethane and a curing agent for the hydrocarbon rubber.
Blends of reactive or functional rubbers with polyurethanes have been prepared prior to this time with the hydrocarbon rubber component being in the major amount (see U.S. Pat. No. 3,514,499; the U.S. Pat. No. to Verdol et al 3,427,366; the U.S. Pat. No. to Massoubre 3,429,948; and the U.S. Pat. No. to Holden 3,562,355, wherein the hydrocarbon rubber and the polyurethane are block copolymerized).
The U.S. Pat. No. to Schollenberger 3,514,499 teaches that when sulfur vulcanizable unsaturated elastomer compounds containing sulfur and an accelerator also contain small amounts of poly(etherurethanes), that the cure time is effectively reduced without apparent adverse consequences on the resulting vulcanizates.
According to Verdol U.S. Pat. No. 3,427,366, general purpose elastomers, especially the ethylene-propylene rubbers, show increased "tack" when modified with urethane elastomers. This is said to be of important commercial significance since one of the major shortcomings of commercially available ethylene-propylene terpolymer rubbers is their inherent lack of tack. It is pointed out in this reference that the polydiene materials have at least 2.1 hydroxyls per molecule which allows cross-linking by means of urethane linkages when a suitable amount of diisocyanate is employed in the reaction. The resultant materials are complex polyolefin-polyurethane rubbers. The present invention contemplates the use of nonreactive hydrocarbon rubbers.
The U.S. Pat. No. to Holden 3,562,355 is concerned with compositions which demonstrate flex cracking resistance and improved resistance to solvent action and ozone comprising a block copolymer together with one or more of three classes of auxiliary polymers; namely, a linear polyester urethane, copolymers of ethylene with esters of terminally ethylenically-unsaturated monocarboxylic acids and saturated aliphatic alcohols and esters of saturated monocarboxylic acids and terminally ethylenically unsaturated aliphatic alcohols. The block copolymers in accordance with the Holden patent are of the general configuration [A-B-A].sub.n wherein n is a whole integer between 1 and 5. If the copolymer is not hydrogenated, the blocks A comprise poly(monovinyl arene) blocks while the block B is a poly(conjugated diene) block. Specific examples of such block copolymers are those having the block configuration of polystyrene-polybutadiene-polystyrene and polystyrene-polyisoprene-polystyrene. The present invention does not contemplate the use of such block copolymers.
The present invention is clearly distinguished from the prior art in that it contemplates a blend of a linear elastomeric polyurethane with a minor amount of a hydrocarbon rubber together with a curing agent for the polyurethane usually a hydroxyl or amino compound, or an amino-alcohol, and a curing agent for the rubber compound, usually a peroxide or sulfur, and wherein the dispersion is of finely divided carbon rubber material in a matrix of polyurethane material, the particle size of the hydrocarbon rubber material being in the submicron range.
Such blends as distinct from copolymers, interreaction products, or block polymers are characterized in that they demonstrate nonmelting properties under conditions of high shear, low hardness, good tear strength, and high modulus. By this invention, therefore, a nonfunctional hydrocarbon rubber can be used to reinforce the physical properties of a polyurethane. Moreover, by being nonfunctional, the hydrocarbon rubber material does not interfere with the apparent pot life of the polyurethane composition. In addition, the presence of the hydrocarbon rubber material both lowers the per pound cost of the formulation and allows cross-linking to be introduced into the system by the use of a peroxide or other hydrocarbon rubber curing agent with no serious degradation of most polyurethane backbones which occur in the cured product.
The only known disadvantage of the invention is that it is difficult to use a peroxide as the curing agent in a polybutadiene blend in a polypropylene oxide/glycol-based polyurethane system because the peroxide tends to degrade the polypropylene oxide backbone. This behavior seems to be specific to the particular polypropylene oxide/glycol-based polyurethanes.